Illumination system for microlithography

ABSTRACT

An illumination system for a microlithography projection exposure apparatus for illuminating an illumination field ( 65 ) with the light from an assigned light source ( 11 ) comprises a pupil shaping unit ( 15, 30 ) for receiving light from the assigned light source ( 11 ) and for generating a predeterminable basic light distribution in a pupil plane ( 31 ) of the illumination system and a transmission filter ( 36 ) assigned to the pupil shaping unit ( 15, 30 ) and having at least one array of individually drivable individual elements for the spatially resolving transmission filtering of the light impinging on the transmission filter in or in proximity to a pupil plane ( 31, 35 ) of the illumination system, the transmission filter ( 36 ) being designed for generating a predeterminable correction of the basic light distribution. An illumination system of this type can generate a multiplicity of location-dependent intensity distributions in a pupil plane of the illumination system, a high transmittance being ensured. The location-dependent intensity distribution in the pupil plane generates an angle-dependent intensity distribution on the illumination field of the illumination system which can be optimized for a mask structure to be imaged.

This application claims priority from German Patent Applications DE 102004 034935.5 filed on Jul. 9, 2004. The disclosures of this patentapplication is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an illumination system for a microlithographyprojection exposure apparatus for illuminating an illumination fieldwith the light from an assigned light source, and to a method forilluminating an illumination field, in particular with such anillumination system.

2. Description of the Related Art

The performance of projection exposure apparatuses for themicrolithographic fabrication of semiconductor components and otherfinely patterned devices is substantially determined by the imagingproperties of the projection objectives. Furthermore, the image qualityand the wafer throughput that can be achieved with the apparatus aresubstantially co-determined by properties of the illumination systemarranged upstream of the projection objective. Said illumination systemmust be able to prepare the light from a primary light source, forexample a laser, with the highest possible efficiency and in the processto generate an intensity distribution that is as uniform as possible inan illumination field of the illumination system.

A high degree of uniformity or homogeneity can be achieved by mixing thelight coming from the light source with the aid of a light mixingdevice. In the case of light mixing devices, a distinction is madeessentially between light mixing devices with fly's eye condensers andlight mixing devices with integrator rods or light mixing rods.

Moreover, it is intended to be possible to set different illuminationmodes at the illumination system in order, for example, to optimize theillumination in accordance with the structures of the individualoriginals to be imaged (masks, reticles). Pupil shaping units areprovided for setting the illumination modes or illumination settings,which pupil shaping units can generate a predeterminable lightdistribution in a pupil plane.

In the case of illumination systems which, for setting differentillumination modes, operate with exchangeable optical elements (e.g.diffractive optical elements or spatial filters), the number ofdifferent illumination settings is limited by the number of differentelements that can be exchanged. If the intention is that a large numberof illumination settings can be set at such systems, a multiplicity ofexchangeable optical elements must be made available, the production ofwhich is complicated and may be associated with considerable costs.

The use of digital filters in illumination systems is known for variouspurposes. The filters may be positioned e.g. as variable aperturediaphragms or as pupil shaping elements for setting a predeterminablelight distribution in a pupil plane. A pupil shaping exclusively bymeans of filters is always associated with loss of light, however,particularly if relatively large regions of the light distributionincident on the filter have to be masked out for producing off-axissettings.

U.S. Pat. No. 6,215,578 describes a wafer stepper comprising anillumination system for generating abaxial illumination settings. Theillumination system has an electronically drivable, first digital filterfor setting illumination settings. Arranged downstream of said firstdigital filter is a second digital filter, which is positioned on theillumination field of the illumination system and generates there apredeterminable mask pattern that is imaged onto a wafer through aprojection objective arranged downstream. Both digital filters can bedriven jointly by means of a computer, thereby enabling the lightdistribution generated by the pupil shaping element to be adapted to themask structure.

U.S. Pat. No. 5,707,501 shows a projection exposure apparatus which, inone embodiment, comprises a transmission filter designed as a liquidcrystal array. Said transmission filter is arranged before a fly's eyecondenser in the illumination system of the projection exposureapparatus and serves as a pupil shaping element in order to set e.g.annular illumination.

JP 06216007 describes a projection exposure apparatus having anillumination system in which a liquid crystal digital filter is arrangedin the region of a pupil plane in the light path behind a fly's eyecondenser. The digital filter is electrically drivable and serves as anadjustable diaphragm for limiting the beam bundle diameter.

JP 10022209 describes a filter which comprises an array of liquidcrystals and is designed for use in an illumination system of aprojection exposure apparatus. The filter serves as a transmissionfilter with location-dependent transmission that can be set variably andpermits the setting of desired two-dimensional intensity distributions.The resolution and the depth of focus of the projection are thusintended to be able to be influenced.

U.S. Pat. No. 6,281,967 shows an illumination system for amicrolithographic projection exposure apparatus having a rod integrator.In the light path before the rod integrator there is provided atransmission filter element for setting the intensity distribution ofthe light falling into the rod integrator. The filter element can beexchanged as required for filter elements having a differenttransmission distribution. A turret plate is arranged in a pupil planebehind the rod integrator, there being fitted in said turret plate aplurality of aperture arrangements which in each case have openings ofdifferent size or form and serve for pupil shaping.

U.S. Pat. No. 6,051,842 describes an illumination system having a fly'seye condenser, upstream of which a variable optical filter is arranged.The filter serves as a variable attenuator for homogenizing the lightdistribution in a field plane upstream of the fly's eye condenser.

The documents JP 12150375, JP 11312639 and JP 11054417 show embodimentsof filters which all serve for homogenizing the light distribution in afield plane. In this case, the location-dependent filter effect isfixedly predetermined and can only be altered by a filter change.

SUMMARY OF THE INVENTION

It is one object of the invention to provide an illumination system fora microlithography projection exposure apparatus which, with a simpleconstruction, permits a high degree of variability in the setting ofillumination modes, a high transmission efficiency of the illuminationsystem remaining ensured. It is another object to provide acorresponding method for illuminating an illumination field, inparticular with such an illumination system.

To address these and other objects the invention, according to oneformulation, provides an illumination system for a microlithographyprojection exposure apparatus for illuminating an illumination fieldwith the light from an assigned light source comprising a pupil shapingunit for receiving light from the assigned light source and forgenerating a predetermined basic light distribution in a pupil plane ofthe illumination system; and a transmission filter assigned to the pupilshaping unit and having at least one array of individually drivableindividual elements for the spatially resolving transmission filteringof the light impinging on the transmission filter, the transmissionfilter being arranged in or in proximity to a pupil plane of theillumination system; the transmission filter being designed forgenerating a predetermined correction of the basic light distribution.

An illumination system according to the invention comprises a pupilshaping unit for receiving light from the assigned light source and forgenerating a predeterminable basic light distribution in a pupil planeof the illumination system, and a transmission filter assigned to thepupil shaping unit and having at least one array of individuallydrivable individual elements for the spatially resolving transmissionfiltering of the light impinging on the transmission filter in or inproximity to a pupil plane of the illumination system. The transmissionfilter is designed for generating a predeterminable correction of thebasic light distribution.

The pupil shaping unit may be designed e.g. for generating conventionalillumination with a different degree of coherence, annular fieldillumination and also dipole or quadrupole illumination in a pupil planeof the illumination system. Such a pupil shaping unit may be designed insuch a way that even in the case of off-axis illumination, only a smallloss of light occurs during the pupil shaping. The basic lightdistribution generated by the pupil shaping unit can be optimizedfurther for a predetermined structure to be imaged on the illuminationfield by the basic light distribution being slightly corrected locallywith the aid of the transmission filter. For this purpose, thetransmission filter can mask out parts of the basic light distributionwhich are not required for the imaging of the predetermined structure,or it can locally introduce a small attenuation in order to exactly seta desired distribution proceeding from the basic light distribution withminimal loss of light.

In one development of the illumination system, the transmission filteris arranged downstream of the pupil shaping unit. The transmissionfilter may be fitted for example in or in proximity to the pupil planeof the illumination system in which the pupil shaping unit generates thebasic light distribution. An arrangement in the region of an opticallyconjugate plane with respect to the pupil plane is also possible.

In one embodiment, the transmission filter is designed or driven in sucha way that it transmits more than 90% and less than 100% of the lightimpinging on it in at least one operating mode, preferably in most or inall of the illumination settings. The transmission filter thus correctsonly a small part of the basic light distribution, so that a large partof the intensity of the illumination radiation is preserved.

In one development of the illumination system, the transmission filteris designed as a digital filter. The individual elements of the digitalfilter can be changed over between two states: approximately completetransmission (approximately 100%) or complete obscuration (0%). Aparticularly simple construction is possible as a result of this. Theindividual elements may also be tunable in order to enable partialtransmission.

In one embodiment of the illumination system, the cross section of theindividual elements is rectangular and the latter are arrangedessentially without an interspace in a manner filling the area in arectangular raster arrangement. The transmission filter can easily bedriven given a regular arrangement of the individual elements. Therectangular raster elements may be square, in particular. The larger thenumber of raster elements, the better the spatial resolution of thetransmission filter effect. However, a large number of raster elementscannot be driven as easily as a small number, so that a compromiseshould be found for the choice of a suitable number of raster elements.Depending on the desired spatial resolution and total area of thetransmission filter, there may be present e.g. between approximately 100and approximately 1,000 raster elements, if appropriate also severalthousand, e.g. between 2,000 and 8,000 or more.

Various principles of action may be utilized for the construction of atransmission filter. The individually drivable individual elements maybe liquid crystal elements, for example, which may belight-nontransmissive (opaque) or light-transmissive (transparent)depending on the electrical driving. It is also possible to set thetransmittance of the individual elements between complete blocking andcomplete transparency in steps or in continuously variable fashion inorder to obtain intermediate values of the transmission (partialtransparency).

A transmission filter may also operate with an array of mechanicalshutter elements that may be configured for example as foldable smalldoors or slides. With mechanical diaphragm elements of this typearranged in rasterlike fashion, a digital filtering is possible in anexpedient manner. For setting intermediate values of the transmission,the beam cross section of the individual part bundles can beindividually reduced to a desired value in order to obtain a desiredtarget value for the transmission desired at the location of thediaphragm element.

Polarization effects may likewise be utilized for the construction of atransmission filter that can be used in the case of the invention. Theapplicant's German patent application 10 2004 011733.0 filed on Mar. 4,2004 shows a transmission filter device having a cell arrangement thatcan be operated in transmission for the purpose of generating alocation-dependent retardation effect on the light of an entrance lightdistribution, which can be driven for the purpose of generating atemporally variable retardation effect, and also at least onepolarization filter arrangement arranged behind the cell arrangement inthe light path. The cell arrangement generates from the entrance lightdistribution a light distribution polarized in location-dependentfashion, on which a polarization-selective, location-dependent intensityfiltering is performed by means of the polarization filter arrangement.If the cell arrangement is designed for setting a temporally variableretardation effect, a time- and location-dependent intensity filteringon the light of the entrance light distribution is possible. Oneembodiment utilizes the electro-optical effect (Pockels effect). Forthis purpose, the individual elements of the transmission filtercomprise suitable electrically drivable nonlinear optical crystals whoseretardation effect on the light passing through can be set incontinuously variable fashion by application of a suitable voltage. Thedisclosure content of this patent application is incorporated in thecontent of the present application by reference.

The examples mentioned above can be operated in transmission, so thatthose components of the impinging light on which no intensity reductiontakes place can pass through the transmission filter essentiallyunimpeded. Reflective variants of transmission filters are alsopossible. By way of example, a transmission filter may comprise an arrayof individually drivable individual mirrors (micromirror array, digitalmirror device, DMD). Such a micromirror arrangement may be incorporatedobliquely into the beam path, so that the impinging light bundle as awhole is deflected by the micromirror arrangement. For spatiallyresolving transmission filtering, individual mirror elements can beadjusted such that the impinging light is deflected into a spatialregion that is no longer registered by the downstream optical subsystemand, consequently, is no longer available in the light path behind thetransmission filter. Such reflective filters are also referred to as“transmission filters” here since they can influence the overalltransmission of the optical system at their incorporation location inspacially resolving fashion.

A spatially resolving transmission filter may also contain atwo-dimensional array arrangement (raster arrangement) of individuallydrivable diffractive optical elements or individually drivableacousto-optical elements which mask out a part of the impingingradiation from the beam path in spacially resolving fashion and can thusact as transmission filters. These transmission filters may be ofreflective or transmissive design.

In one development of the illumination system, the pupil shaping unitcomprises at least one diffractive optical element. The diffractiveoptical element is usually positioned in a field plane of theillumination system, where it generates an angular distribution of theillumination light which generates a spatial distribution of theintensity upon transfer to a downstream pupil plane.

In one development of the illumination system, the pupil shaping unitcomprises a changing device for exchanging the diffractive opticalelement for at least one further diffractive optical element. Variousbasic light distributions can be selected by means of exchangingdiffractive elements. Since the number of basic light distributions thatcan be set in this case is limited by the number of exchangeablediffractive elements and the production thereof is complicated, thetransmission filter is used for fine tuning of the illuminationradiation to the mask structures to be illuminated.

In one embodiment of the illumination system, the pupil shaping unitcomprises an axicon system. Adjustable axicons can be used to realizeoff-axis illumination settings (e.g. annular illumination) without anyloss of light.

In one development of the illumination system, the pupil shaping unitcomprises a zoom unit. The zoom unit serves for scalemagnifying/demagnifying of the light distribution generated. Incombination with the axicon and the diffractive optical element, thepupil shaping unit can generate a plurality of basic light distributionspractically without any loss of light. The pupil shaping unit may beconstructed e.g. in the manner as described in the applicant's EP 0 747772. The content thereof is incorporated in the content of thedescription by reference.

In one embodiment, an integrator rod arrangement having at least oneintegrator rod is provided behind the pupil shaping unit and thetransmission filter. The integrator rod serves for homogenizing a lightdistribution that enters it. The pupil shaping unit generates a basiclight distribution which is corrected by the transmission filter beforea homogenization of the illumination light takes place.

In one development of the illumination system, an integrator rodarrangement having at least one integrator rod is provided between thepupil shaping unit and the transmission filter. The pupil shaping unitgenerates a basic light distribution which is subsequently homogenizedby the rod. This homogenized light distribution is corrected by means ofthe transmission filter.

In one embodiment of the illumination system, the cross-sectional areaof the filter elements is small relative to a parcel area produced bythe integrator rod in a pupil plane of the illumination system. Thisproves to be advantageous if the light distribution homogenized by theintegrator rod is intended to be corrected by means of the transmissionfilter, since a spatially resolved filtering effect is possible in thiscase, too within individual parcels.

In one development of the illumination system, a refractive opticalelement for generating a light distribution adapted to the form of theillumination field is provided in or in proximity to a pupil plane. Inthe case of systems with an integrator rod, the usually rectangular formof the illumination field corresponds to the form of the cross-sectionalarea of the rod, i.e. the aspect ratio matches. Through an imagingobjective arranged downstream of the integrator rod, the rod exitsurface can be imaged onto the illumination field in a size altered bythe imaging scale of the imaging objective.

In one embodiment of the illumination system, a fly's eye condenser withat least one raster arrangement of raster elements is arranged behindthe pupil shaping unit. Fly's eye condensers of customary designcomprise a raster arrangement positioned in a first plane and a rasterarrangement positioned in a downstream second plane, which is conjugatewith respect to the first plane. The transmission filter may bepositioned e.g. in proximity to the first raster arrangement before thelatter.

In one embodiment of the illumination system, neither a fly's eyecondenser nor an integrator rod arrangement is fitted behind the pupilshaping unit. Such illumination systems, which are typically used inwafer scanners, often operate with diaphragms or filters as homogenizingunits. This is possible since homogenizing the intensity integrated overthe illumination field may suffice in the scanning process.

In one development of the illumination system, a control unit isprovided for adapting the basic light distribution generated by thepupil shaping unit and the transmission function of the transmissionfilter to a mask structure to be illuminated. An optimum adaptation ofthe illumination radiation to the mask structure to be illuminated mayhave an advantageous effect on the exposure quality of themicrolithography projection exposure apparatus.

In a method according to the invention for illuminating an illuminationfield, which may be carried out in particular with an illuminationsystem described above, the light distribution in a pupil plane of theillumination system is adapted to a mask structure to be illuminated bya procedure in which, by means of a pupil shaping unit, a basic lightdistribution is set in or in proximity to a pupil plane and atransmission filter positioned in or in proximity to a pupil plane or ina plane that is conjugate with respect thereto is driven for the purposeof filtering a part of the light distribution that is not required forthe illumination of the mask structure. The driving is preferablyeffected in such a way that the filter transmits more than 90% of theintensity of the basic light distribution. The method makes it possibleto precisely adapt a light distribution in a pupil plane to a maskstructure to be imaged without significant reduction in the illuminationintensity occurring. In individual cases, lower overall transmissionsmay also be useful, e.g. 75%, 80% or 85%.

In one development of the method, one from a plurality of diffractiveoptical elements is selected for the purpose of generating the basiclight distribution. Various basic light distributions can be set byexchanging diffractive optical elements.

In one refinement of the method, a change is carried out between a firstand a second mask structure, a light distribution adapted to the firstmask structure being converted into a light distribution adapted to thesecond mask structure by changing the transmission function of thetransmission filter, in particular without a change of the diffractiveoptical element being carried out. In the case of two mask structuresthat are illuminated with similar light distributions, it may beexpedient for an adaption of the light distribution to be carried outexclusively by means of the transmission filter.

The above and further features emerge not only from the claims but alsofrom the description and from the drawings, in which case the individualfeatures may be realized, and may represent advantageous embodimentsprotectable per se, in each case on their own or as a plurality in theform of subcombinations in embodiments of the invention and in otherfields.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic side view of an embodiment of an illuminationsystem for a microlitho-graphy projection exposure apparatus with apupil shaping unit and a transmission filter;

FIG. 2 shows a schematic detail view of the illumination system fromFIG. 1;

FIG. 3 shows a schematic detail view of an illumination system in whichthe integrator rod is exchanged for a fly's eye condenser; and

FIG. 4 shows examples of the optimization and correction of basic lightdistributions with the aid of transmission filters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an example of an illumination system 10 of a projectionexposure apparatus for microlithography which can be used in thefabrication of semiconductor components and other finely patterneddevices and operates with light from the deep ultraviolet range in orderto obtain resolutions down to fractions of micrometers. The light source11 used is an F₂ excimer laser having an operating wavelength ofapproximately 157 nm, the light beam of which is oriented coaxially withrespect to the optical axis 12 of the illumination system. Other UVlight sources, for example ArF excimer lasers having an operatingwavelength of 193 nm, KrF excimer lasers having an operating wavelengthof 248 nm or mercury vapor lamps having an operating wavelength of 365nm or 436 nm or light sources having wavelengths of less than 157 nm arelikewise possible.

The light from the light source 11 firstly enters a beam expander 13,which expands the laser beam and forms an expanded profile with parallellight from the original beam profile. In the light path behind the beamexpander 13, a diffractive optical element 15 is arranged in a fieldplane 14 of the illumination system. This forms, together with a zoomaxicon objective 30 positioned behind it in the beam path, a pupilshaping unit serving for generating a predeterminable basic lightdistribution in an exit pupil 31 of the objective 30. A refractiveoptical raster element 32 is arranged in the exit pupil 31. The exitpupil 31 is also referred to hereinafter as pupil shaping surface 31 ofthe illumination system. The basic light distribution in the pupilshaping surface 31 can be set by adjusting the zoom axicon objective 30and also by exchanging the diffractive optical element 15 by means of achanging device 16 (cf. e.g. EP 0 747 772).

By way of example, conventional illumination with a different degree ofcoherence, or an approximate annular field, dipole or quadrupoleillumination can be set as basic light distributions by means of thepupil shaping unit 31. Particularly in the case of off-axisillumination, the basic light distribution may be distributednon-uniformly over illuminated and unilluminated partial regions of thepupil shaping surface, which may result in greater intensity variations.

A transmission filter 36 is provided in the beam path directly beforethe refractive optical raster element 32. Said filter serves forcorrecting the basic light distribution generated by the pupil shapingunit 15, 30. The transmission filter 36 has a multiplicity of individualdrivable optical channels arranged in a raster arrangement, so that thetwo-dimensional transmission function of the transmission filter can beset variably in location-dependent fashion.

A coupling-in optic 40 arranged behind the pupil shaping surface 31transmits the light from the pupil shaping surface 31 onto therectangular entrance surface 44 of a rod-type light integrator 45produced from synthetic quartz glass (or calcium fluoride), which lightintegrator mixes and homogenizes the light passing through by means ofmultiple internal reflection. The pupil shaping surface 31 is aFourier-transformed plane with respect to the entrance surface 44, sothat a spatial intensity distribution in plane 31 is transformed into anangular distribution at the rod entrance 44. An intermediate field plane47 lies directly at the exit surface 46 of the rod 45, in which plane isarranged a reticle masking system (REMA) 50, which serves as anadjustable field diaphragm. The downstream objective 55 images theintermediate field plane 47 with the masking system 50 onto a plane 65,which is also referred to here as reticle plane. A reticle 66 isarranged in the reticle plane 65. The plane 47 of the reticle maskingsystem and the reticle plane 65 are planes in which an illuminationfield of the illumination system is situated. The reticle plane 65coincides with the object plane of a projection objective 67, whichimages the reticle pattern into its image plane 68, in which a wafer 69coated with a photoresist layer is arranged. The objective 55 contains afirst lens group 56, a pupil intermediate plane 57, into which filtersor diaphragms can be introduced, a second and a third lens group 58, 59and a deflection mirror 60 situated in between, which mirror makes itpossible to incorporate the large illumination device horizontally andto mount the reticle horizontally. The integrator rod 45 generates aplurality of parcels in the pupil surface 57 of the objective 55 bymeans of the multiple reflections of the light in its interior.

The illumination system 10 forms together with the projection objective67, an adjustable reticle holder, which holds the reticle 66 in theobject plane 65 of the projection objective, and an adjustable waferholder, a projection exposure apparatus for the microlithographicfabrication of electronic devices but also of diffractive opticalelements and other micropatterned parts. The illumination system can beused both in a wafer stepper and in a wafer scanner.

The angular distribution which the illumination system 10 generates inthe illumination field 65 is determined by prescribing thelocation-dependent intensity distribution on the pupil shaping surface31. If said angular distribution is to be adapted to a predeterminedreticle 66, the basic light distribution can be set by exchanging thediffractive optical element 15 and/or setting the zoom axicon objective30 essentially without any loss of light. If the basic lightdistribution is already set optimally, the transmission filter can alsobe switched “to passage” (full transmission). A slight modification ofthe basic light distribution for precise adaptation to the reticle 66 isoften desired, however, which can be carried out by setting thetransmission filter 36. The loss of light occurring during the filteringcan be kept within small limits since the pupil shaping unit 15, 30 setsthe intensity distribution in the pupil shaping surface 31 essentiallycorrectly. The combination of exchangeable diffractive optical elements15, zoom axicon objective 30 and transmission filter 36 thereforepermits the generation of a multiplicity of different lightdistributions in the pupil shaping surface 31 with only little loss oflight and a very precise optimization of the two-dimensional intensitydistribution in the pupil shaping surface 31.

In order to optimally adapt the radiation supplied by the illuminationsystem to the reticle 66, a control unit 35 is provided, which isconnected to the changing unit 16, the zoom axicon objective 30 and alsothe transmission filter 36.

The transmission filter 36 is designed as a digital filter. It hasindividual elements (pixels) with a rectangular cross section which arearranged in a manner filling the area in a rectangular rasterarrangement (LCD display). The individual elements of the digital filtercan be in two states, corresponding to virtually complete transmissionof the illumination light or complete obscuration thereof. The crosssection of the individual elements is designed such that it is less thanthe parcel cross section of the parcels generated by the integrator rod45 in the pupil plane 57. The cross section of the individual elementsmay be e.g. 1/n_(E) of the parcel cross section in a specific direction,where n_(E)=2, 3, 4, 5 . . . . The parcel cross section Δx in the xdirection is a function of the length L, the refractive index n and alsothe rod extent x of the rod integrator in the x direction considered andalso the focal length f_(E) of the coupling-in optic and can becalculated by Δx=2·f_(E)·n·sin (arctan(x/(2·L)). The same appliescorrespondingly to the y direction.

In the example shown, the integrator rod 45 has a rectangular crosssection of 35 mm by 11 mm and a length of approximately 570 mm. Thecross-sectional area required on the pupil shaping surface 31 in orderto cover a parcel is 2.148 mm by 0.614 mm. If nine pixels of thetransmission filter 36 are to cover a parcel area in order to achieve agood spatial resolution, each pixel should have a cross section of 0.716mm by 0.205 mm. If the illuminated part of the pupil shaping surface 31has a diameter of 100 mm, then a number of approximately 6000 pixels arerequired for the transmission filter 31.

The transmission filter 36 may, as shown in FIG. 2, be positioneddirectly upstream of the refractive optical element 32 in a plane 36 aor directly downstream of said optical element in a plane 36 b. It mustbe taken into consideration that the beam divergence downstream of therefractive optical element 32 is greater than the beam divergenceupstream thereof. Therefore, a positioning upstream of the refractiveoptical element is generally more favorable. As an alternative, thetransmission filter may also be arranged in a pupil plane downstream ofthe integrator rod 45, e.g. in the pupil plane of the REMA objective 57.It is also possible for more than one transmission filter to be arrangedin the illumination system 10; however, it must be taken intoconsideration that the pixels of digital transmission filters, even ifthey are switched to transmission, absorb a small part of the light.Therefore, it is preferred to use only a single transmission filter.

An illumination system according to the invention may also be equippedwith a fly's eye condenser 75 as a homogenizing unit, as shown in FIG.3. In this case, the transmission filter may be arranged in a plane 37 adirectly before the fly's eye condenser 75 or in a plane 37 b directlybehind the fly's eye condenser. These planes lie in proximity to pupilplanes of the illumination system, so that it is possible to influencethe spatial distribution of the intensity in a pupil plane.

An illumination system according to the invention may also have neithera rod integrator nor a fly's eye condenser. In this case, it is possibleto use e.g. diaphragms or filters for homogenization.

In FIG. 4, two examples are used to elucidate how a basic lightdistribution can be optimized or corrected with the aid of an embodimentof a digital filter 36 shown schematically. For this purpose, subfigures(a) to (d) in each case schematically show, by means of grey-hatchedareas, a basic light distribution in the manner of a quadrupoledistribution in which four trapezoidal illumination regions with highintensity are present which are arranged outside the optical axis 12 andlie opposite one another in pairs and the remaining region of the pupilplane and thus, in particular, also the region of the optical axis isunilluminated. The transmission filter 36 is represented by arectangular raster with a multiplicity of squares lying one beside theother, bright squares representing a raster element switched totransmission and black squares representing a raster element switched toblocking. In FIG. 4(a), the entire transmission filter is switched totransmission. In FIG. 4(b), the basic light distribution set by thepupil shaping unit remains unchanged, but the individual cells lying atthe lateral edges of the trapezoidal illumination regions are switchedon, so that the oblique edges of the trapezoids are masked out. Thisresults in illumination regions which are in each case narrower in thetransverse direction than without this optimization by the transmissionfilter. Less than 10% of the area of the transmission filter is maskedout for this correction.

In the case of the basic light distribution shown in FIG. 4(c), thetrapezoidal poles of the quadrupole distribution in the pupil shapingsurface that lie in the y direction have a larger area than the poleslying in the x direction. This may be favorable in the case of specificreticle structures which have different line widths and/or spacings fordifferent structure directions. However, if a largely matchingillumination in the x and y directions is desired, then this can beachieved, proceeding from the basic light distribution shown in FIG.4(c), with the aid of the transmission filter by virtue of the factthat, in the case of the larger illumination regions lying in the ydirection, the edge regions thereof are masked out to such an extentthat the size and form of the poles then remaining essentially match thesize and form of the illumination poles in the x direction (see FIG.4(d)). In this way, the transmission filter can be utilized for thecorrection of asymmetrical distributions.

The invention has been explained by way of example on the basis ofillumination systems in which the pupil shaping unit comprises, interalia, a zoom axicon objective in conjunction with a diffractive opticalelement. The invention can also be realized with pupil shaping units ofdifferent design. Such illumination systems are shown for example in thepatent applications with publication number WO 2005/026843 A2, WO2004/006021 A2, U.S. 2004/0119961 A1 or U.S. 2003/0086524 A1 or patentsU.S. Pat. No. 6,658,084 B2, U.S. Pat. No. 6,611,574 B2 from theapplicant.

The invention is not restricted to the field of microlithography. By wayof example, illumination systems in microscopy may also be configured inaccordance with the invention.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention ant its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. The applicant seeks, therefore, tocover all such changes and modifications as fall within the spirit andscope of the invention, as defined by the appended claims, andequivalents thereof.

1. An illumination system for a microlithography projection exposureapparatus for illuminating an illumination field with the light from anassigned light source comprising: a pupil shaping unit for receivinglight from the assigned light source and for generating a predeterminedbasic light distribution in a pupil plane of the illumination system;and a transmission filter assigned to the pupil shaping unit and havingat least one array of individually drivable individual elements for thespatially resolving transmission filtering of the light impinging on thetransmission filter, the transmission filter being arranged in or inproximity to a pupil plane of the illumination system; the transmissionfilter being designed for generating a predetermined correction of thebasic light distribution.
 2. The illumination system as claimed in claim1, wherein the transmission filter is arranged downstream of the pupilshaping unit.
 3. The illumination system as claimed in claim 1, whereinthe transmission filter is one of designed and driven in such a way thatit transmits more than 90% and less than 100% of the light impinging onthe transmission filter in at least one operating mode.
 4. Theillumination system as claimed in claim 1, wherein the transmissionfilter is designed as a digital filter.
 5. The illumination system asclaimed in claim 1, wherein a cross section of the individual elementsis rectangular and the individual elements are arranged essentiallywithout an interspace in a manner filling the area in a rectangularraster arrangement.
 6. The illumination system as claimed in claim 1,wherein the transmission filter comprises a two-dimensional array ofliquid crystal individual elements.
 7. The illumination system asclaimed in claim 1, wherein the transmission filter is designed as areflective transmission filter, the individual elements being formed byindividually drivable individual mirrors for optionally deflectingpartial bundles of the impinging light from the beam path.
 8. Theillumination system as claimed in claim 1, wherein the transmissionfilter has a cell arrangement that is operated in transmission for thepurpose of generating a location-dependent retardation effect on thelight of an entrance light distribution, which is driven for the purposeof generating a temporally variable retardation effect, and also atleast one polarization filter arrangement arranged behind the cellarrangement in the light path.
 9. The illumination system as claimed inclaim 1, wherein the transmission filter has individual elements in theform of at least one of diffractive optical elements and opto-acousticoptical elements.
 10. The illumination system as claimed in claim 1,wherein the pupil shaping unit comprises at least one diffractiveoptical element.
 11. The illumination system as claimed in claim 10,wherein the pupil shaping unit comprises a changing device forexchanging the diffractive optical element for at least one furtherdiffractive optical element.
 12. The illumination system as claimed inclaim 1, wherein the pupil shaping unit comprises an axicon system. 13.The illumination system as claimed in claim 1, wherein the pupil shapingunit comprises a zoom unit.
 14. The illumination system as claimed inclaim 1, wherein an integrator rod arrangement having at least oneintegrator rod is provided downstream of the pupil shaping unit and thetransmission filter.
 15. The illumination system as claimed in claim 1,wherein an integrator rod arrangement having at least one integrator rodis provided between the pupil shaping unit and the transmission filter.16. The illumination system as claimed in claim 1, wherein an integratorrod arrangement having at least one integrator rod is provided andwherein a cross-sectional area of the filter elements is small relativeto a parcel area produced by the integrator rod in a pupil plane of theillumination system.
 17. The illumination system as claimed in claim 1,wherein a refractive optical raster element for generating a lightdistribution adapted to the form of the illumination field is providedin or in proximity to a pupil plane (31) of the illumination system. 18.The illumination system as claimed in claim 1, wherein a fly's eyecondenser with at least one raster arrangement of raster elements isarranged behind the pupil shaping unit.
 19. The illumination system asclaimed in claim 1, wherein neither a fly's eye condenser nor anintegrator rod arrangement is fitted downstream of the pupil shapingunit.
 20. The illumination system as claimed in claim 1, wherein acontrol unit is provided for adapting the basic light distributiongenerated by the pupil shaping unit and a transmission function of thetransmission filter to a mask structure to be illuminated.
 21. Amicrolithography projection exposure apparatus comprising at least oneillumination system as claimed in claim
 1. 22. A method for illuminatingan illumination field, wherein a light distribution in a pupil plane ofan illumination system is adapted to a mask structure to be illuminatedby a procedure in which a basic light distribution is set in or inproximity to the pupil plane and a transmission filter positioned in orin proximity to that pupil plane or an optically conjugate plane isdriven for the purpose of filtering a part of the basic lightdistribution that is not required for the illumination of the maskstructure.
 23. The method as claimed in claim 22, wherein the driving iseffected such that the transmission filter transmits more than 90% andless than 100% of the intensity of the basic light distribution.
 24. Themethod as claimed in claim 22, wherein one from a plurality ofdiffractive optical elements is selected for the purpose of generatingthe basic light distribution.
 25. The method as claimed in claim 24,wherein a change is carried out between a first and a second maskstructure, a light distribution adapted to the first mask structurebeing converted into a light distribution adapted to the second maskstructure by changing the transmission function of the transmissionfilter, in particular without a change of the diffractive opticalelement being carried out.